a) Field of the Invention
The present invention relates to a magnetic sensor including a magnetic sensing element such as magnetoresistance element or the like and particularly to such a magnetic sensor suitable for use in the detection of minute magnetic patterns which are printed on paper bank notes or other paper-like media.
b) Description of the Prior Art
Magnetic sensors are known which use various types of magnetic sensing elements. One of these known magnetic sensing elements is a magnetoresistance element. The magnetoresistance element is one that has a variable electrical resistance depending on change of the magnetic flux density. For example, such a magnetoresistance element may be formed by a semiconductor having a relatively large mobility, such as Si, InSb, InAs, GaAs or the like. FIG. 1 shows the schematic arrangement of a magnetic sensor constructed in accordance with the prior art. The magnetic sensor of the prior art uses magnetic sensing elements in the form of a magnetoresistance element, the output of which is used to ascertain the authenticity of paper bank notes.
The magnetic sensor comprises two magnetoresistance elements MR1 and MR2, a magnet 10, a holder 12 and a casing 14. The magnetoresistance elements MR1 and MR2 are normally disposed on a substrate (not shown) and connected to each other to form such a circuit as shown in FIG. 2. The magnetoresistance elements MR1 and MR2 (and actually their substrate) are held by the holder 12 with the magnet 10, or are mounted on the magnet 10. The holder 12 includes a plurality of terminals not shown which are embedded therein. DC voltage is applied to the magnetoresistance elements MR1 and MR2 through these terminals. The output voltage of the magnetoresistance elements MR1 and MR2 is also taken out through the terminals. It is of course possible that the magnetoresistance elements MR1 and MR2 may be mounted on the magnet 10 directly or without the use of the substrate.
The magnetic sensor may be used to sense minute magnetic materials and the like which are formed on paper bank notes or other paper-like media. More particularly, when the magnetoresistance elements MR1 and MR2 are connected with their directions of anisotropy being opposite to each other with a DC voltage Vin being applied across these elements as shown in FIG. 2 and if a medium 16 having a magnetic pattern is moved across a space above the magnetoresistance elements MR1 and MR2, such a voltage Vout as shown in FIG. 3 will be outputted from the junction point between the magnetoresistance elements MR1 and MR2.
The voltage Vout is one that is variable about a neutral potential being a DC potential which is determined depending on the magnetoresistance elements MR1 and MR2. The voltage Vout varies depending on a change of the electrical resistance in the magnetoresistance elements MR1 and MR2. As the medium 16 including the magnetic materials on the surface thereof is moved above the magnetoresistance elements MR1 and MR2, the magnetic fluxes crossing the magnetoresistance elements MR1 and MR2 vary in density. As a result, the electrical resistance in the magnetoresistance elements MR1 and MR2 will also vary. Thus, the magnetic sensor can sense the medium 16 including the magnetic materials printed from a magnetic ink as in paper bank notes or the like through the changing Vout due to changes of the electrical resistance in the magnetoresistance elements MR1 and MR2. The magnet 10 serves as means for magnetically biasing the magnetoresistance elements MR1 and MR2 on detection of the magnetic pattern, thereby improving the magnetic sensor in detection sensitivity.
The resolution of the magnetic sensor depends on the pitch between the magnetoresistance elements MR1 and MR2 or the distance L between the magnetoresistance elements MR1, MR2 and the medium 16. If the flux in the magnet 10 is not divergent, the resolution is equal to the pitch between the magnetoresistance elements MR1 and MR2. However, the resolution is lower than the pitch between the magnetoresistance elements MR1 and MR2 since the flux of the magnet 10 is actually divergent.
The detection sensitivity of the magnetic sensor depends on the distance L between the magnetoresistance elements MR1, MR2 and the medium 16. More particularly, the detection sensitivity decreases as the distance L increases. Thus, the reliability on read-out will decrease if the distance L is not constant.
To maintain the resolution and detection sensitivity and to secure the reliability on read-out, the magnetic sensor requires means for maintaining the distance L between the magnetoresistance elements MR1, MR2 and the medium 16 constant. The casing 14 (and more particularly its upper air gap) provides such a means. More particularly, the distance L can be maintained substantially constant even when the medium 16 is moved along the top of the casing 14 since the top of the casing 14 is positioned to maintain the spacing between the casing top and the magnetoresistance elements MR1, MR2 constant. Since the medium 16 is in direct contact with the casing 14, but not with the magnetoresistance elements MR1 and MR2, any collision or friction will not be produced between the magnetoresistance elements MR1, MR2 and the medium 16 even if the speed of movement of the medium 16 is increased.
In such an arrangement of the prior art, however, it is difficult to improve the resolution of the magnetic sensor and to reduce the size of the same. First, the resolution is restrained by the distance L. Second, the entire magnetic sensor is increased in size since it is required to increase the size of the magnet so that it will be influenced by the divergence of the magnetic flux as little as possible. Finally, the resolution and/or detection sensitivity tend to be irregular.